This invention relates to the reclamation of raw metal from machine shop scrap and more particularly relates to heating such scrap in a rotating tumbler barrel to promote the oxidation and vaporization of water, oil and other adulterants from machine shop cuttings.
In foundries and machine shops, metal cuttings are produced during boring, milling, and other operations. Water and various oils, which are used in performing these operations, adhere to the scrap cuttings. The type of scrap obtained from these operations is herein collectively referred to as chips. The water, oil, and other materials which adhere to the chips are collectively referred to as adulterants. It is the broad purpose of my invention to separate the adulterants from the chips and to recover suitably pure metal for further processing and reuse.
A known way of cleaning the adjulterants from the chips is to heat the chips to a temperature which is sufficient to vaporize some adulterants and promote the oxidation of others. This has conveniently been done by passing the chips through a rotating tumbler barrel which is inclined somewhat to the horizontal. Chips are fed into the upper end of the tumbler barrel and are tumbled with the aid of gravity to the lower end of the barrel. A burner is located at the lower end of the barrel which passes hot gases over the chips and heats them. Conventionally, after the heat has effected the necessary vaporization and oxidation, the chips are cooled until such time as they are to be melted and reused.
There are several problems and inefficiencies present in the above system. Often they arise because designers are forced to make trade-offs and compromises between various design factors.
The first problem is the oxidation of the chip metal itself. Although oxidation of the adulterants is desirable, oxidation of the chips produces corrosive losses and impedes melting. Designers of heretofore known systems have had two options. They could heat the chips in an oxygen free atmosphere and thereby prevent chip oxidation. Obviously, this inhibits the desirable oxidation of the adulterants. In the alternative, designers could heat the chips in the presence of oxygen if the chips were not heated unacceptably hot to a temperature at which they readily oxidize. For example, bronze chips heated to 400.degree. F. will not substantially oxidize while at the same time adulterants will vaporize and oxidize. However, it is clear that the production rate could be greatly increased if a way were found to permit heating of the chips to a temperature nearer their melting point, which for some types of bronze is approximately 1,800.degree. F., without producing significant oxidation of the metal.
One solution, offered by MacDonald in U.S. Pat. No. 2,852,418 is to introduce the chips, the combustion fuel and some excess air all into the tumbler barrel at its upper end. These components must be properly proportioned so that the oxygen in the air is consumed by oxidation of the adulterants before the chip metal becomes hot enough to substantially and readily oxidize. This system unfortunately results in an inefficient and costly use of fuel. As combustion gases and chips move toward the lower outlet end of the barrel, the gases are cooled and the chips are heated. The chips in this inefficient system are never elevated to the temperature of the burner flame. The combustion gases are emitted into the atmosphere from the lower end of the barrel when they are at the same temperature as that to which the chips are raised, thereby wasting a significant portion of the heat capacity of the fuel.
If a counterflow principle were used in the MacDonald system so that the chips entered the upper end of the tumbler barrel and the fuel and excess air entered the lower end, fuel use would be more efficient because the combustion gases would leave the barrel from its upper end at a lower temperature. However, the chips would then be hottest and therefore most likely to oxidize where the excess air was the most abundant; that is, at the lower end where the air was introduced into the barrel.
Another problem with the above described counterflow system is that, even if the excess air were not intentionally introduced, air would be drawn into or aspirated into the tumbler barrel where the stationary burner abuts the rotating tumbler barrel.
Conventionally, cleaned chips, after being emitted from the tumbler barrel, are cooled and stored in bins. When stored as chips, substantial oxidation often occurs because with a mass of chips a large surface area of metal is exposed to air contact. The presence of oxide on the chip surfaces tends to act as an insulator between chips and inhibit melting of the chips. Even if the oxide were not present, heat conduction through a mass of chips is poor because of the small area of contact between chips and the large quantity of air between the chips.
Finally, it is very inefficient to heat chips for cleaning purposes, then cool them, and then reheat them in the melting process.
It is therefore an object of my invention to provide an improved method and apparatus for cleaning and preparing scrap metal for reuse.
Another object of my invention is to permit in one system the use of the efficient counterflow system of heating chips in a tumbler barrel, the introduction of an oxidizing agent into the barrel and the raising of the chips to a temperature near their melting temperature in the absence of oxygen.
Another object of my invention is to prevent the aspiration of air into a tumbler barrel at the connection of a burner to the barrel.
Another object of my invention is to eliminate the oxidation of cleaned chips and to facilitate the subsequent melting of cleaned chips.
Another object of my invention is to use most efficiently the heat capacity of the heating fuel.
Still another object of my invention is to provide a tumbler barrel which has interior walls which uniformly distribute the tumbling chips within the gas passing through the barrel.
Further objects and features of my invention will be apparent from the following specification and claims when considered in connection with the accompanying drawings illustrating several embodiments of my invention.